Eighty years ago, we solidly had eight planets in the Solar System: the same eight we have now.

But in the late 1920s, a young astronomer was looking up at the sky, night after night, searching for tiny moving objects that could possibly be planets out beyond Neptune. Using this technique of looking at a patch of sky repeatedly over the course of a week, Clyde Tombaugh searched for moving objects, finding many comets and asteroids, but — like everyone else — found no signs of a new planet.

Until January of 1930. I’ve managed to dig up Clyde Tombaugh’s slides of those two fateful nights that led to the discovery of Pluto.

See where the white arrow is pointing? That’s what Clyde Tombaugh noticed. This faint, tiny object appeared to move, night after night, against the fixed background of stars. Very quickly, this discovery was announced and confirmed, and on February 18th, 1930 (four score and one day ago), it was declared that our Solar System has nine, and not eight planets!

Today, we know that Pluto is not some long-lost frozen world out beyond the gas giants, but rather the first object ever discovered in the Kuiper Belt, the largest structure in our Solar System consisting of thousands upon thousands of tiny frozen worlds, ranging in size from little comets to huge, Moon-sized worlds!

The IAU may tell us that Pluto isn’t a planet anymore, but don’t let a small group of scientists tell you what is and isn’t of great importance in our Solar System. Pluto was the only known Kuiper Belt object for 48 years, and do you know what the second one discovered was?

Charon, Pluto’s giant Moon! (As imaged here by the Hubble Space telescope, and shown along with Pluto.) We didn’t discover a third one until the 1990s, and now we have a deep-space probe on its way to Pluto to roam among the Kuiper Belt, for the sole purpose of scientific exploration.

So happy 80th birthday, Pluto. I may be in the scientific minority, but I still think of you as a giant in our Solar System, and as the ninth classic planet.

Good ol’ blink comparators – they’re great for finding stuff (too good if you’re looking at 2 scenes with substantial differences). These days we just let computers do the job of finding changes; no more straining those human eyes.

Neptune was discovered on September 23, 1846. This meant almost 90 years passed between the discovery of the eighth and outermost planet and Tombaugh’s discovery of Pluto. The amount of determination and sheer hours Tombaugh spent to find Pluto made the story of Pluto’s discovery compelling to me as a kid, esp. since no other planet had been discovered in the half century from 1930 to the 1980s. The story of Pluto’s discovery by one lone, determined guy in a very cold observatory night after night is part of what first got me interested in astronomy.

Our solar system does NOT have only eight planets, and it is far more than a minority of scientists who still view Pluto as a planet and a Kuiper Belt Object. The same is true for Haumea, Makemake, and Eris. . Only four percent of the IAU voted on this, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet. Under this definition, our solar system has 13 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.

Does anyone else realize that around 1830 there were 13 planets in the solar system?
Anyone want to know WHY that was?… it was because astronomers were discovering the first few asteroids. The Kuiper belt of icy objects beyond Neptune is the late 20th century’s version of the Asteroid belt.
They just didn’t have a cute dog to name the damn asteroids after in the early 1800s, so now Americans (& only Americans) are upset about it.
Here is the largest of the planet-asteroids:http://en.wikipedia.org/wiki/Ceres_(asteroid)

Happy Birthday, Ninth Planet! Pluto will always be a planet for me, whatever some cabal of Foreignji (probably intent on some ‘postcolonial’ project of diminishing the Western-described solar system) at the IAU may think. Pluto has that cool fused-PL symbol, and it was the subject of Don Wollheim’s “Secret of the Ninth Planet”, a favourite book of my Long-Ago Youth. And, for Lovecraft fans, Pluto will always be Yuggoth, home of the nightgaunts. Pluto also appeared in the forgotten proto-anime “Space Angel” back in the early Sixties. So— I’ll never give up Pluto as Planet IX! Happy birthday, little guy! (And your moon Charon, too!)

It is not only Americans who reject the IAU’s controversial demotion of Pluto. I have been running a blog advocating Pluto’s reinstatement for three-and-a-half years, and during that time, I have heard from people all over the world who oppose the demotion. A lot of these are people on astronomy forums or members of astronomy-related groups online, people who already have a strong interest in the solar system.

The demotion of Ceres from planet to asteroid was actually a mistake. Nineteenth-century astronomers could not resolve Ceres into a disk, so they didn’t know that unlike almost all other objects in the asteroid belt, Ceres is round. This means it is large enough to be in hydrostatic equilibrium. The fact that it is spherical was not recognized until the late 19th century. Pallas and Vesta are questionable, as they appear to have been spherical and subsequently hit by asteroids that knocked part of them. Pluto and the other spherical KBOs are as unlike the majority of tiny, shapeless KBOs as Ceres is unlike the majority of asteroids in the belt between Mars and Jupiter. This means that both demotions, of Ceres, and of Pluto, were wrong.

And why is it a problem if we have a large number of planets in the solar system? Isn’t a desire to artificially keep the number low a sentimental rather than a scientific argument?

I still have to get used to naming Pluto as the 9th planet. It has glued into my mind that Pluto was the 8th planet growing up after all these years. Funny how facts can be altered with scientific advancements and discoveries. I have to agree with the last message of this post though – Pluto may be one of the smallest galaxy structures but she will always be a giant cornerstone in the final frontier that we call space.

I can’t believe it’s Pluto’s 80th birthday. Having said this, the planet has yet to be visited by a spacecraft. We do know that the planet is mostly made up of ice which is one of the reasons it might be very difficult for any mission to take off.

Its pluto’s 80th birthday, I guess I didn’t know that until I read this. I have done some research on the universe but never really looked into the age of the planets. Maybe I will look into that next. Thanks for letting me share.Alarm California

The IAU may tell us that Pluto isn’t a planet anymore, but don’t let a small group of scientists tell you what is and isn’t of great importance in our Solar System. Pluto was the only known Kuiper Belt object for 48 years, and do you know what the second one discovered was?spread betting explained

I love studying outer space. This has been a interest for me for a very long time. I think Pluto is interesting because it is so far away from us. It amazing to think we know so much about this planet.

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I hear that yesterday a Jupiter sized planet has been spotted that was once part of another solar system, now swallowed up by the milky way. I’ll bet we’ll be hearing of more new discoveries as telescope technology improves. Does anyone know if the the universe is continuing to spread out though?

Though Seewald now interviews the Pope himself, the journalist “pulls no punches,” posing some of the thorniest questions any pope has had to address. Believers and unbelievers will be fascinated to hear Benedict’s thoughtful, straightforward, and thought-provoking replies. This is no stern sermon or ponderous theological tract, but a lively, fast-paced, challenging, even entertaining exchange.

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Pluto ,Happy birthday from side as well !!!! I hope you don’t mind being 80 as people older people are more wiser.I don’t want to drown in a discussion that you are a 9th or the 8th planet .The important thing for me is ,you are still up there and revolving as happily as you started 80 years ago.

Pluto, the last celestial mass in the solar system is not a planet really as pointed out recently. The nine planets that exist in the solar system still consists of nine planets excluding Pluto. Likely, there are more out there needs to be discovered.DEWALT DW087K